The present invention relates to inexpensive sintered metal parts, which have improved solid lubricating effects such as excellent machinability, wear resistance and plastic workability, and to producing them.
As well known so far in the art, powder compact-forming properties, machinability, wear resistance, slidability and other properties are imparted to sintered metal parts or their sintering precursors by adding to them solid lubricants such as graphite, molybdenum disulfide, boron nitride and talc.
Of the above-mentioned solid lubricants, talc in particular has been reported to serve well as both a binder for forming metal powders and a lubricant, as set forth in Claus G. Goetzel, "TREATISE ON POWDER METALLURGY", Vol. 1, pp. 254-256 (1949).
More illustratively, materials obtained by forming and sintering iron or ferroalloy powders having a talc content of 0.1 to 5% by volume are known to have improved free cutting properties to increase the service life of cutting tools, as disclosed in Japanese Patent Laid-Open No. 63(1988)-93842.
As set forth in Japanese Patent publication No. 63(1988)-118047, sintered metal parts made by forming and sintering metal powders having a talc content of 0.1 to 10% by weight are known to have improved compression denseness, machinability and other properties and increased resistance to sticking wear, thus lending themselves well fit for internal combustion engine valve seats and guides.
Thus, sintered metal parts obtained by forming and sintering mixed metal powders containing talc have a major advantage of being not only improved in terms of machinability and wear resistance but limited in terms of friction resistance to molds when compacted as well. Their another merit is that the price of talc itself is relatively law little.
Since talc powders--Mg.sub.3 Si.sub.4 O.sub.10 (OH).sub.2 --contain some moisture, however, when compacts of mixed metal powers containing such talc powders are sintered, it is likely that talc might suffer dehydration and decomposition in the course of heating, increasing the dew points of reducing atmospheric gases.
For that reason, unless sintering conditions are placed under strict control in obtaining sintered metal parts of sophisticated geometries such as internal combustion engine valve seats and guides, it may then be impossible to impart desired properties to the resulting sinterings.
Especially where such compacts are continuously fed in large quantities into a sintering furnace, the intra-furnace gases are contaminated so severely that care should be taken to reduce the amount of talc added, increase the quantity of the intrafurnace gases supplied or use some special-purpose sintering furnaces, resulting in necessity of additional processes.
In addition, talc is broken down into magnesium metasilicate--MgSiO.sub.3 and hard silicon dioxide--SiO.sub.2 in the course of sintering by heating. This appears to have been caused by a silicon dioxide. Sintered metal parts obtained by adding talc to them may attack or abrade the associated sliding members.
One object of this invention is to provide a sintered metal part well suited for sintered metal slugs, etc. used especially for producing internal combustion engine valve seats or guides, oil-impregnated bearings, gears and other members by means of plastic working, and a method for making such a metal part.
The present invention is also directed to an iron-based sintered sliding member which is best suited for various bearings and built up of a sintered alloy improved in terms of wear resistance and free cutting properties.
So far, wear-resistant sliding members suitable for various purposes have been available, which are formed of an iron-based sintering material containing a large amount of solid lubricating graphite so as to impart wear resistance thereto and in which a part of the graphite has been diffused throughout the matrix in the course of sintering and allowed to remain as free graphite after sintering so as to make use of the lubrication of the residual graphite.
In order to obtain free graphite, however, this sintering-material has to be sintered at a temperature lower than that applied to generally available iron-based materials, resulting in a decrease of intercrystalline bond strength and so making the sliding member likely to wear away when used under high pressure conditions.
In some efforts to eliminate the defect of the above-mentioned iron-based sintered sliding members, it has been proposed to sinter a sintering feed material containing given amounts of copper, tin and phosphorus in a liquid phase at 980.degree. to 1100.degree., thereby dispersing free graphite throughout a pearlite structure containing a ternary Fe-P-C eutectic (steadite phase) in a mixed form. In this regard, see Japanese Patent Publication No. 54(1979)-42335 or Japanese Patent Publication No. 55(1980)-34858.
The thus improved iron-based sintered sliding members have been shown to enhance the strength of the sintering feed rather than to lower it even upon sintered at a relatively low temperature; they lend themselves well fit for sliding parts used under high pressure conditions, like bearings. However, they have a grave defect of being poor in free cutting properties and leave much to be desired in this regard.
Thus, another object of this invention is to provide an iron-based sintered sliding member built up of a sintered alloy excelling in wear resistance, free cutting properties and other properties, which can be used even under high pressure condition and is primarily improved in terms of machinability over the conventional sintered sliding member.